Question

I have a program that is trying to use create and cancel through an implemented pool.

The creation is as follows:

int threadsNum=10;
while (created<threadsNum){
    pthread_t newThread;
    pthread_struct *st; //Open the thread that handle the deleting of the sessions timeout.
    st = (pthread_struct*)malloc(sizeof(pthread_struct));
    st->id = created;
    st->t = newThread;
    pthread_mutex_lock( &mutex_threadsPool );
    readingThreadsPool[created] = st;
    pthread_mutex_unlock( &mutex_threadsPool );
        if((threadRes1 = pthread_create( &newThread, NULL, pcapReadingThread, (void*)created)))
        {
        syslog(LOG_CRIT, "Creating Pcap-Reading Thread %d  failed.",created); 
                printf( "Creating Pcap-Reading Thread %d  failed.\n",created);
                exit(1);
        }
    syslog(LOG_INFO, "Created Pcap-Reading Thread %d Successfully.",created); 
    created++;
}

Later I try to cancel them and restart them :

    pthread_t t;
pthread_struct* tstr;
int i;
pthread_mutex_unlock( &mutex_threadsPool );
//first go on array and kill all threads
for(i = 0; i<threadsNum ; i++ ){
    tstr = readingThreadsPool[i];
    if (tstr!=NULL){
        t = tstr->t;
        if (pthread_cancel(t)!=0){
            perror("ERROR : Could not kill thread");
        }
        else{
            printf("Killed Thread %d \n",i);
        }
    }
}

So far so good, but the only problem is that the output is
Error : Could not kill thread : Illegal Seek
Killed Thread 1

Killed Thread 2

Killed Thread 3

Killed Thread 4

Killed Thread 5

Killed Thread 6

Killed Thread 7

Killed Thread 8

Killed Thread 9

Why doesn’t it kill the thread in the 0 index as well?

And I couldn’t find anything about Illegal Seek..

Thanks for your help people

Thanks

Answer 1

The problem is that newThread is being used before it is initialized:

pthread_t newThread;
pthread_struct *st;
st = (pthread_struct*)malloc(sizeof(pthread_struct));
st->id = created;
st->t = newThread;

but newThread does not receive a value until after the successful invocation of pthread_create(). It appears that newThread variable is retaining its previous value on subsequent iterations of the loop, which results in the correct cancelling of all threads except for last thread started as its id is never inserted into the readingThreadsPool array.

You need to populate the st->t member after the call to pthread_create().

As the code currently stands, it is possible for an entry to be inserted into the readingThreadsPool array even though it is not actually a thread yet. Put the insertion logic after the call to pthread_create():

if((threadRes1 =
        pthread_create(&(st->t), NULL, pcapReadingThread, (void*)created)))
{
    syslog(LOG_CRIT, "Creating Pcap-Reading Thread %d  failed.",created); 
    printf( "Creating Pcap-Reading Thread %d  failed.\n",created);
    exit(1);
}
pthread_mutex_lock( &mutex_threadsPool );
readingThreadsPool[created] = st;
pthread_mutex_unlock( &mutex_threadsPool );

or if the pcapReadingThread() function accesses readingThreadsPool and expects an entry for itself (which I think might be the case due to created being passed) then enclose the pthread_create() inside the lock of mutex_threadsPool.